Auto merge of #4837 - alexcrichton:failed-submodule-checkout, r=matklad

Fix updating submodules past failures

If a submodule-of-a-submodule failed to update then Cargo the next time
around wouldn't automatically retry updating the next submodule. This commit
fixes that by ensuring that if a parent git repository looks updated we still
recurse into its own submodules to ensure they're all updated.

cc #4071